Microelectronic capacitors are widely used in microelectronic devices. For example, microelectronic capacitors are widely used to store information in a Dynamic Random Access Memory (DRAM).
As the integration density of DRAMs continues to increase, the surface area of a memory cell tends to decrease. This may cause a decrease the capacitance of the cell capacitor, which may result in a lower performance and increased soft error rate. Therefore, it is generally desirable to maintain a large cell capacitance, notwithstanding the decreasing size of the DRAM cell.
Many techniques have been used to increase the capacitance in a given unit area. In particular, as is well known, capacitance C of a capacitor is given by: ##EQU1## where .di-elect cons..sub.0 is the dielectric constant of free space, .di-elect cons..sub.r is the relative dielectric constant of a dielectric film, A is the effective area of an electrode, and d is the thickness of the dielectric film. Accordingly, from the above equation, the capacitance can be increased by varying one or more of three parameters: the dielectric constant of the dielectric film, the effective area of the capacitor and/or the thickness of the dielectric film.
It has been proposed to increase the effective area of the capacitor by increasing the effective area of a capacitor electrode. In particular, a capacitor electrode with ridges and valleys may be formed to thereby increase the surface area of the electrode. It has been proposed to use Hemispherical Grain (HSG) silicon film having a rugged surface. This may be used in combination with a three dimensional capacitor structure such as a stack, a trench and/or a cylindrical structure to increase the effective area of the electrode per unit area of the microelectronic substrate.
U.S. Pat. No. 5,385,863 to Tatsumi discloses a technique for increasing the effective area of the capacitor electrode using an HSG silicon film. In particular, a capacitor electrode of polysilicon film is formed. The polysilicon film is formed by depositing an amorphous silicon film on an insulating film covering a substrate, generating a plurality of crystal nuclei on the amorphous silicon film and growing the crystal nuclei into mushroom or hemisphere-shaped crystal grains, thereby converting the amorphous silicon film into the polysilicon film.
Unfortunately, it may be difficult to maintain the amorphous film in a clean condition. Contamination of the surface by foreign materials or crystallization of an area of the amorphous silicon film may suppress the surface migration of the silicon atoms in the amorphous silicon film, and may thus reduce or prevent crystal nucleation and growth. Accordingly, poor quality HSG films may be produced.
FIGS. 1A and 1B are scanning electron microscope (SEM) photos showing the result of forming HSG films on a partially crystallized amorphous silicon film on a semiconductor substrate. As noted from the figures, HSGs are normally formed on amorphous silicon, while no growth of nuclei is observed in a crystallized portion due to the absence of activation energy of silicon.
Similarly, when the amorphous silicon surface is contaminated by foreign materials and thus the amorphous silicon atoms are combined with foreign atoms, it may be difficult for the silicon to migrate. The amorphous silicon surface thus may be further contaminated, and crystal nucleation and growth may end if the foreign materials are accumulated to a predetermined thickness.
A publication by H. Watanabe et al. entitled "A New Cylindrical Capacitor Using Hemispherical Grained Si (HSG--Si) for 256MB DRAMs", IEDM, 1992, pp. 259-262, describes the fabrication of a cylindrical electrode structure of a p-doped amorphous silicon film. A native oxide on the electrode surface is removed by a diluted HF solution. HSG--Si is then formed on the amorphous silicon surface using seeding method, Si.sub.2 H.sub.6 molecule irradiation and annealing at 580.degree. C. in an ultra-high vacuum chamber.